The bug

The test mentioned in the title has been failing to build since #95922, but the underlying issue existed beforehand and was only exposed by a bugfix in iputils.
Last tested on a332da8.

Incorrect behaviour, identical before and after the iputils bump

When the router machine attempts to ping 2001:db8::, it gets replies from itself. Why is completely unclear to me, but the route it obtains for this address looks suspicious to me:

# ip r g 2001:db8::
anycast 2001:db8:: from :: dev lo table local proto kernel src 2001:db8::ffff:0:0:5eb6 metric 0 pref medium

The source address looks right. The use of anycast and the lo device does not.

The difference

The new failure is due to a bug fix in iputils's ping: "discard packets with wrong source address (comply RFC 1122, commit: 5e052ad)" in iputils (release notes)

The solution

I have no idea. I'm not familiar enough with prefix delegation matters to understand why this is happening, nor how to fix it. cc @andir, author of the test.